The invention relates to the tracking of signals with at least one subcarrier such as Binary Offset Carrier (BOC) modulated signals. Particularly, the present invention relates to tracking of BOC (modulated) signals of a GNSS (Global Navigation Satellite System).
In next generation GNSSs, BOC modulations and multiplexed BOC (MBOC) modulations will be used. Examples are the Galileo E1 open signal, a CBOC (6,1,1/11) signal (Composite BOC using a sine subcarrier), the Galileo PRS (Public Regulated Service) signals on E1 and E6, a BOCc(15,2.5) signal and a BOCc(10,5) signal, respectively, and the GPS (Global Positioning System) M-code, a BOC(10,5) signal. In more general terms, the above mentioned signals may be referred to as subcarrier modulated signals. Such subcarrier modulated signals comprise a carrier signal, which is modulated with a pseudo random noise (PRN) code, and which is additionally modulated with one or more subcarriers. Additionally, navigation message data may or may not be modulated onto the carrier signal.
A BOC modulated signal without the subcarrier modulation corresponds to a Binary Phase Shift Keying (BPSK) signal used like it is used for GPS SPS (Standard Positioning Service), which has a triangular autocorrelation function. FIG. 1 shows an example subcarrier signal 101, 102 having a subcarrier symbol duration
            T      s        =          1              2        ⁢                  f          s                      ,wherein fs is the subcarrier rate. FIG. 1 also illustrates the symbol duration
      T    c    =      1          f      c      of a symbol of the PRN code (wherein fc is the code rate), yet the PRN code signal itself is not shown in this figure. In the illustrated example, the subcarrier rate fs is twice as high as the code rate fc and the resulting BOC signal is referred to as a BOC(2m,m) signal (based on the notation BOC(m,n) where the respective frequencies are given by fs=m·1.023 MHz, fc=n·1.023 MHz). The code rate fc may also be referred to as the chip rate and a symbol of the PRN code (having a code symbol duration Tc) may be referred to as a chip. The subcarrier 101, 102 itself has a saw-tooth like autocorrelation function 103 as shown in FIG. 1 for a BOC(10,5) signal. The autocorrelation function 113 of a BOC signal is approximately given by the multiplication of a triangular PRN-code autocorrelation function 123 with the subcarrier autocorrelation function 103. Therefore, this autocorrelation function 113 has multiple peaks as shown in FIG. 2 for a BOC cos(10,5) signal.
The autocorrelation function 113 which exhibits multiple peaks has advantages and disadvantages: The main peak 114 of the autocorrelation function 113 is significantly narrower than the single peak of the autocorrelation function 123 of the corresponding BPSK signal. This offers the potential for an increased tracking accuracy, i.e., less code jitter, and a better multipath performance. In particular, this may be used for determining (i.e., for tracking) a transmission delay of the BOC signals with increased accuracy. The transmission delay of the BOC signal may then be used for determining the position of a GNSS receiver.
On the other hand, a tracking loop may lock to a side peak 115 instead of the main peak 114. If the locking to a side peak 115 is not recognized and corrected, systematic errors in the pseudo-range measurements occur, which in turn lead to position errors. For a BOC cos(15,2.5) (also referred to as BOCc(15,2.5)) modulated signal, i.e., a BOC modulated signal using a subcarrier which is phase shifted by 90 degrees with respect to the PRN code, a false lock to the first side peak 115 leads to a pseudo-range error of approximately 10 meter, and for a BOC cos(10,5) modulated signal, this error is approximately 15 meters. However, it is also possible that a tracking loop locks to a side peak 115 further away from the main peak 114, such that the resulting error is a multiple of the error for a false lock to the first side peak 115.
Different techniques have been proposed for tracking these signals, e.g., bump jumping, a Sidelobe Cancellation Method, BPSK-like techniques, a Multiple Gate Delay discriminator, and a Double Estimator technique to name a few.
The US patent application US 2010/0104046 A1 describes the Double Estimator technique, which consists of three independent but cooperative loops for carrier, subcarrier, and code. The Double Estimator provides two independent delay estimates, one from code tracking, and one from subcarrier tracking. The code tracking delay estimate τ is less accurate, while the subcarrier tracking delay estimate τ* is ambiguous with the subcarrier chip duration Ts. The final delay estimate τfinal is then calculated by resolving the subcarrier delay ambiguity using the less accurate code delay estimate, for example by rounding the difference of code delay and subcarrier delay, both divided by the subcarrier chip duration, to the nearest integer and adding the result multiplied by the subcarrier chip duration to the subcarrier delay:
      τ    final    =            τ      *        +          round      ⁢                          ⁢                        (                                    τ              -                              τ                *                                                    T              S                                )                ·                  T          S                    
Another approach is the Double Phase Estimator, described in the publication “Double Phase Estimator Towards a New Perception of the Subcarrier Component”, D. Borio, InsideGNSS, May/June 2015, http://www.insidegnss.com, available under http://www.insidegnss.com/auto/mayjune15-WP.pdf. The difference between the Double Estimator and the Double Phase Estimator is that the Double Phase Estimator uses an arctan discriminator like it is used in PLLs (Phase Lock Loops) for subcarrier tracking.
A further approach is described in the European patent application EP13290093.7. The tracking method disclosed in this application employs two independent but cooperative loops: One loop is used for carrier tracking, which is similar to a conventional PLL or FLL (Frequency Lock Loop). The other loop is performing a subcarrier tracking using an early-minus-late discriminator. Additionally, the subcarrier loop NCO (Numerical Controlled Oscillator) produces two replica signals composed of a prompt subcarrier, multiplied in one case with an early code replica, in the other case multiplied with a late code replica. These two replicas are correlated with the incoming signal, and the two correlation results are provided to an early-minus-late discriminator for detection of false locks to a side-peak of the subcarrier lock loop.